The rapid growth of the subscriber base in GSM has stressed the need for increased voice capacity. Thus, both mobile network operators and telecom equipment manufacturers have agreed to open a new study item in 3GPP standardization. The study item has been named MUROS (Multiple User Reusing One Slot) and is described in GP-072027, “WID on MUROS”. The basic idea is to allow two (or more) users to share the same carrier frequency and the same timeslot, both in the downlink and in the uplink. There are several possible technical realizations of MUROS.
In one MUROS solution (GP-070214, “Voice Capacity Evolution with Orthogonal Sub Channel”, source Nokia), a QPSK modulation is used in the DL (downlink). The two user signals are mapped to the real and imaginary parts of the baseband signal. These are called the I and Q sub-channels, and under some conditions they are orthogonal, and therefore named OSC (Orthogonal Sub-Channels). In PCT/SE2008/050116 it is proposed to use a hybrid quaternary modulation in the DL.
In the uplink, i.e. in case data are transmitted from a mobile station to a base station, one solution (GP-070214, “Voice Capacity Evolution with Orthogonal Sub Channel”, source Nokia) is to use GMSK modulation in the uplink. The two GMSK modulated signals from two mobile stations are transmitted on the same timeslot and carrier frequency (or sequence of frequencies in case frequency hopping is deployed). On the receiver side, multi-user detection or interference cancellation techniques can be used to demodulate the two signals.
Even though legacy mobiles are supported by the technique described in the above referred PCT application, new MS (Mobile Station) types will still be required because a new training sequence set is introduced.
Two (or more) users share one radio resource. Even though the name of the concept implies that orthogonality is involved, the two users are in fact not perfectly orthogonal since time dispersion on the channel (due to multipath propagation on the radio channel and filters in the transmitter and receiver) will cause leakage between the I and Q sub-channels. For the downlink, this means the two users will interfere one another. For the uplink, i.e. when data are transmitted from the mobile station to the base station, the phase difference between the two users will be random and hence orthogonality is not achieved even in the absence of time dispersion.
In the existing solutions, this lack of orthogonality results in that each user is interfered by the user using the other sub-channel. This interference will degrade the performance of each user. If discontinuous transmission (DTX) is used, the inter-sub-channel interferer will sometimes be present and sometimes not. The performance (e.g., speech quality) will be limited by the time periods when the interferer is present.
Further, in a scenario in which OSC is used for only a subset of the channels in a cell (e.g., due to the current cell load, there is no need to multiplex two users on all channels), users on OSC channels will experience worse link quality (e.g., coverage) than users on non-shared channels. This makes it more difficult to manage the quality in the network.
In the existing OSC solution, the frequency hopping standardized for GSM (3GPP TS 45.002, “Multiplexing and multiple access on the radio path”) can be applied to the QPSK modulated signal in downlink and each of the GMSK modulated signals in uplink. Consequently, the two sub-channels will use the same frequency hopping sequence and hence the same frequency and timeslot at any given instant in time.
In summary, the existing solutions have several drawbacks leading to a deteriorated speech quality.